Chromium oxide coating for siliceous articles

ABSTRACT

AN IMPROVED TREATMENT FOR PROVIDING A CHROMIUM OXIDE COATING ON SILICEOUS ARTICLES, PARTICULARLY, HIGH SILICA CONTENT FIBERS, AND ARTICLES COMPRISED THEREOF, BY APPLYING A SOLUTION OF A WERNER COMPLEX OF AN ORGANIC DICARBOXYLIC ACID AND A DICHROMATE, PARTICULARLY AMMONIUM   DIOXALATODIAQUOCHROMATE, TO THE ARTICLE AND HEATING THE ARTICLE TO PYROLITICALLY DECOMPOSE THE COMPLEX TO FORM SAID COATING.

April 1973 B. E. CARUSO 3,725,117

CHROMIUM OXIDE COATING FOR SILICEOUS ARTICLES Filed Sept. 5. 1970 I O PRIOR ART PROCESS; |.3%Cr O O PRESENT PRoc ss; 2.4% cr o A UNTREATED 1D LL 0.6- E :3 E)

QZ 1 1 I l o 200 400 600 800 looo' I200 I300 TEMPERATURE (F) INVENTOR.

BERNARD E.cARus0 ATTORNEYS.

United States Patent 3,725,117 CHROMIUM OXIDE COATING FOR SILICEOUS ARTICLES Bernard E. Caruso, Upland, Califi, assignor to Hitco, Gardena, Calif. Filed Sept. 3, 1970, Ser. No. 69,288 Int. Cl. C03c 25/02 US. Cl. 117-100 S 18 Claims ABSTRACT OF THE DISCLOSURE An improved treatment for providing a chromium oxide coating on siliceous articles, particularly, high silica content fibers, and articles comprised thereof, by applying a solution of a Werner complex of an organic d1- carboxylic acid and a dichromate, particularly ammonium dioxalatodiaquochromate, to the article and heating the article to pyrolitically decompose the complex to form said coating.

BACKGROUND OF THE INVENTION (1) Field of the invention The present invention relates to treatment of siliceous articles and, more particularly, this invention relates to an improved method of applying a chromium (III) oxide coating on the surfaces of high silica content fibers.

(2) Description of the prior art Siliceous textiles have been treated to form a chromium sesquioxide bonded layer on the surface of the constituent fibers to improve high temperature resistance while maintaining breaking strength, softness and flexibility characteristics.

In one prior art treatment, the textile to be coated 1S dipped first in a solution of chromium (III) sulfate and then in a solution of aluminum hydroxide. These solutions react, causing the precipitation of a chromium (III) hydroxide coating on the surface of the constituent fibers. The coated textile is washed and dried and is then heated to pyrolitically decompose the hydroxide to form chromium (III) oxide. The resultant coating has been found to be excessively dusty and non-uniform. This non-uniformity is particularly evident in woven broad goods, where the subsurface fibers of each yarn thread and the portions of the surface fibers of the constituent warp and fill yarns lying at the crossover points of these yarns, remain essentially uncoated. In addition, the ammonium hydroxide treating solution is objectionable to handle, and the chromium (III) sulfate solution requires heating and stirring for solution. Furthermore, the technique requires separate dipping vats and a wash bath.

In another prior art treatment, the textile to be coated is dipped in a solution of an organic chromuim salt, such as chromic acetate, chromic formate or chromium naphthenate. The coated textile is dried and is then heated to pyrolytically decompose the organic chromium salt to form chromium (III) oxide. As in the aforementioned prior art treatment, the resultant coating is non-uniform. In addition, the coated textile is stiff; subsequent flexing, while eliminating this stiffness, results in some fiber damage. Furthermore, the organic chromium salt solution requires heating and stirring for solution.

OBJECTS AND SUMMARY OF THE INVENTION It is therefore an object of this invention to provide an improved method of applying a chromium oxide coating to siliceous articles.

A further object of the invention is to provide a simple, effective, economical and rapid method of improving the high temperature stability of silica fibers.

Yet another object of the invention is the provision 3,725,117 Patented Apr. 3, 1973 of relatively dust-free, higher strength siliceous textiles, essentially all of the interior and exterior component fibers of which contain a uniform chromium oxide coating over substantially their entire surface.

These and other objects and many other attendant advantages of the invention will become apparent as the description proceeds.

In accordance with the invention a simplified, rapid, effective and less costly method of coating and bonding a protective chromium oxide layer on siliceous articles is accomplished by substantially coating the article with a treating agent comprising a Werner complex of an organic dicarboxylic acid and an inorganic dichromate salt. The article coated with said complex is then heated to pyrolytically decompose the complex, ultimately forming a bonded layer of chromium (III) oxide. The resultant coating is uniform, has excellent adherence and penetration and is relatively dust-free.

The invention will become better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of an apparatus for practicing the method of the invention; and

FIG. 2 is a set of curves comparing the thermal conductivity of samples prepared according to the method of the invention and according to prior techniques as a function of temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring more particularly now to the steps of the present method, the siliceous article to be treated is contacted with and impregnated by a selected treating agent in accordance to the invention. The contacting can be carried out in any suitable manner, for example, by spraying or painting the surface, or by immersing the article in a bath of the treating agent. The article is maintained in contact with the agent for a suflicient period of time to cause it to be completely wetted by the treating agent. This can usually be accomplished in a relatively short period of time, for example, less than one minute depending upon the size of the article and the speed at which the agent is applied.

The wet article, saturated with a solution of the treating agent, is then fired to decompose the agent and bond the chromium oxide layer to the surface. Optionally the wet article may be dried before firing. Firing is preferably conducted in air. Heating may be at a temperature from about 1000 F. to about 3000 F. to pyrolytically decompose and bond the agent to the surface. Heating may be conducted by various means such as radio frequency or infrared heating, or by insertion of the treated article in a furnace. A very practical continuous process practiced for treating continuous lengths of filament or fabric is illustrated in FIG. 1.

Referring now to FIG. 1, a continuous treating process is illustrated for a siliceous fabric 10 which is fed from a stock roll 12 past immersion rolls 14 and 16 which are immersed below the level of a solution 18 of treating agent contained Within a shallow dipping tank 20. The wet fabric 22 passes around guard rolls 24 and 26 and then enters an open air furnace 28. The furnace is preferably maintained at a temperature of about 1500 to 2000 F., with a fabric speed of about 3 to 12 feet per minute, and a firing time of about 5 to 20 minutes, preferably about 10 minutes at 1700 F.

The fabric dries and is fired to cause its component fibers to contain a bonded coating of chromium oxide as it leaves the furnace 28. It is wound up on take-up roll 30. The treatment in accordance to the invention results in 3 application of about 1 to 6% chromium oxide by weight to the article, preferably about 2 to 4% chromium oxide. High silica content materials have been employed in the form of fibers, woven textiles, rovings, tape, sleeving, yarns, cordage, bonded powder, foam, powder, microballoons, sheets, batts and flake.

Such articles containing from about 96% to 99% or more by weight of silica, have a number of superior characteristics which particularly adapt them for use in high temperature environments. They have relatively high softening points, are stable chemically even at high temperatures, and have particularly good high temperature ablative properties, i.e., erosion resistance.

These articles have found wide acceptance in aircraft, industrial and missile fields. High silica content fibers and textiles have been used as a reinforcement in laminated or other types of solid components for use in jet engines, rockets and other reentry vehicles which are exposed to extremely high temperature gases. Structures including these fibers and textiles have been exposed to temperatures as high as 15,000 F. for very short periods of time and have substantially retained their structural integrity.

Moreover, high silica content fibers have also been employed in the form of felted materials and bulk batts and the like, in thermal insulating blankets, filtration elements, reboilers, transpirational cooling and liquid transport applications.

Such high silica content fibers can be prepared from glass fibers in any suitable manner, such as that set forth in US. Pat. No. 2,491,761 to Parker et al. The starting materials for such fibers are glasses, preferably high alumina, high lime content glasses although other glasses can be used. The usual preparation of the silica fibers involves leaching of glass fibers in a suitable acid leaching agent, such as hydrochloric, sulphuric, nitric, acetic, chloroacetic or similar acid which removes glass-forming oxides other than silica but does not substantially attack silica. Accordingly, hydrofluoric, fluosilicic acid and the like are not used.

After the fibers are removed from an aqueous solution of the acid in the leached form, that is, stripped of nonsiliceous oxides, they are washed free of residual acid, and fired at, for example, 1800 F.2000 F. in order to drive off water of hydration and to shrink the fibers to their finished form.

A typical analysis of these fibers is as follows:

Percent sio 99.3 Tio 0.38 A1203 0.18 Zr O 0.02 B203 0.07 CaO 0.01 AgO 0.01 Na O 0.01 All other nil.

Fiber diameters range from about 0.00003 inch to about 0.0004 inch depending upon the use. The individual fibers can be formed into multifilament threads or yarns which are woven into fabrics.

Despite the advantageous properties of high silica content articles, even the highest purity articles containing substantially more than 99% silica exhibit substantial decreases in breaking strength when exposed to temperatures of about 2000 F.-2600 F. for appreciable lengths of time. A particularly pronounced decrease in breaking strength is observed, for example, upon exposure of high purity Woven silica fabric to temperatures of 2400 F. or above for 10 to 20 minutes or longer. Moreover, the high silica content fabric under such circumstances becomes brittle and stiff, rather than remaining soft and flexible. This depreciation of physical characteristics is believed to be due to at least partial devitrification of the component fibers of the yarn.

The process of the invention may be practiced on any siliceous article but is most useful in treating high silica content articles to increase the strength and high temperature resistance and stability of these articles. For example, the treatment of high purity woven silica fabric in accordance with the invention has been found to lower thermal conductivity of the fabric by 10% while maintaining the soft flexible characteristics thereof without embrittlement. The finished product has a greenish oast due to the fact that the chromia is in the form of chromium sesquioxide (Cr O bonded to the surface of the component fibers. This substantially continuous protective layer of chromium sesquioxide is capable of effectively preventing devitrification of the silica fibers so that they can withstand temperatures as high as about 2700 F. for longer periods of time than uncoated fibers. And, except for improvements in high temperature stability, thermal conductivity, and emissivity, the treated fabric is otherwise similar in properties to untreated fabric. Thus, the flexibility, drapability, tensile strength, feel, and most other properties before exposure to elevated temperature remain the same. Moreover, although most mineral acids cause some mild discoloration of the treated fabric, they do not markedly affect the properties thereof.

The treating agent utilized in accordance with the present invention is an organic chromium-based Werner complex formed by the reaction of ammonium-dichromate, or precursor thereof, such as ammonium chromate, with a dicarboxylic acid or precursor thereof, such as the acid anhydride or acyl chloride.

For example, a dipping solution can be prepared by adding ammonium dichromate and oxalic acid dihydrate to room'temperature deionized water. These chemicals when added to water in a stoichiometric ratio of 1:7, react according to the following:

Ammonium Oxalie acid dichromate dihydrate 2[(C 04)2C1'(Hz0)2]NH 6C 02 17H2O Ammonium dioxalatodiaquochromate The reaction is spontaneous, very vigorous, and exothermic. The carbon dioxide evolved provides thorough mixing, and within approximately 30 minutes the starting compounds are dissolved and reacted without stirring, heating or otherwise attending to the reaction. The solution may be green colored initially, but upon cooling, the result is a violet colored solution of cis-ammonium dioxalatodiaquochrornate of the formula:

It is preferred to maintain a stoichiometric excess of oxalic acid, since ammonium dichromate is found to stiffen fabric to which it is applied presumably by binding together its component fibers. Thus, the ratio of the acid dihydrate to the dichromate is at least about 5:1, preferably about 811 to assure complete reaction of the dichromate. On a weight basis this ratio is about 4: 1.

The total concentration of the reactants depends on the desired weight percent of chromium oxide coating. To apply a chromium oxide coating of about 2 weight percent to C--48 Refrasil cloth (HITCO), the dipping solution should be prepared with one pound of ammonium dichromate and four pounds of oxalic acid dihydrate per gallon of deionized water. Refrasil C-l00-48 is a harness weave cloth of thread formed from 99% silica fibers. The cloth has a nominal width of 33 inches, nominal thickness of 0.035 inch and a nominal weight of 18.5 oz./ yd. C10048 cloth contains about 59 warp threads per inch and 40 fill threads per inch. About 6 gallons of solution is sufficient for treatment of 100 yards of this fabric, exclusive of the solution necessary to fill the dipping tank to a minimum operational level.

The following examples are offered solely for purposes of illustration and are not intended to in any way limit the invention:

EXAMPLE 1 Samples of Refrasil C-10048 cloth were dipped in 0.25%, 1%, 4%, 5%, and 10% by weight aqueous solutions of ammonium dichromate. Half of each sample Was oven dried at 220 F. and then fired 10 minutes at 1000 F. The remaining halves were fired 10 minutes at 1000 F. without any preliminary drying. All samples were stiff and covered with a black coating. They were then fired 10 minutes at 1700 F., whereupon they were all covered with a green chromium sesquioxide coating. However, the samples were still stiff. The stilfness could be eliminated by flexing the samples, but this method of unbinding the fibers resulted in damage to them, thereby reducing the fabric strength.

EXAMPLES 2-5 Table I lists the weight percent of ammonium dichromate and oxalic acid dihydrate added to deionized water to prepare the solutions used in these examples, as well as the calculated oxalic acid dihydrate: ammonium dichromate equivalent ratios. In these examples, the ammonium dichromate was first dissolved in the deionized water, and the oxalic acid dihydrate was then slowly added and allowed to react with the ammonium dichromate, without heating or stirring. As the oxalic acid dihydrate was added, the solutions darkened, gas was evolved, and the oxalic acid dihydrate dissolved. Samples of C-l48 Refrasil were dipped in the solutions and then fired 10 minutes at 1000 F. The samples prepared in Examples 2 and 3 were both equally stilt after firing, but not as still as the samples prepared in Example l. The samples prepared in Examples 4 and 5 showed no apparent stiffness after firing.

An additional sample of C-10048 Refrasil was dipped for 30 seconds in the violet-colored solution prepared in Example 5. It was oven dried one hour at 220 F.; after 5 minutes in the oven it changed color from violet to bluegreen and became still. After completion of the drying, microscopic examination showed that the coating had penetrated the fiber bundles well, and that the internal fibers of each thread were coated with an excess of a crystalline substance.

The sample was then placed in an oven at 270 F. for

minutes. No changes were observed. A test run on a piece of the sample showed the blue-green coating to be water soluble. The sample was then placed in a furnace at 700 F. for two minutes; after 90 seconds in the furnace the sample was flexible and black in color. Microscopic examination revealed that the fibers were still covered with an excess of individual crystals, and groups of crystals. The sample was placed in a furnace at 1000 F. for 5 minutes. It was completely limp and silvery black in color. No decrease in strength was noticed, and the crystal deposits observed previously appeared unchanged. When the sample was then placed in a furnace at 1200 1300 F. for 5 minutes it became silvery green in color, indicating the formation of chromium sesquioxide, and was still completely flexible.

6 EXAMPLE 7 Twenty-five pounds of ammonium dichromate were added to 50 gallons of deionized water contained in a large polyethylene-lined drum. One-hundred pounds of oxalic acid dihydrate were then added slowly over a period of about one hour. The vigor with which the spontaneous reaction proceeds was much more apparent than when it was conducted on a laboratory scale. The re actants were completely dissolved and reacted within approximately 30 minutes after addition of the final increment of oxalic acid dihydrate, and a green-colored solution was formed. This green-colored solution was not encountered in previous laboratory-scale preparations. It is believed to have resulted from the higher solution temperature achieved in the present example; this was a result of the heat evolved by the reaction being dissipated more slowly by the larger volume of solution of the present example. The green solution was allowed to cool overnight, whereupon its color changed to violet, indicative of cis-ammonium dioxalatodiaquochromate. The violet solution was then applied to a roll of B grade C--48 Refrasil cloth in the apparatus of FIG. I. The furnace was at a temperature of 1700 F. Processing speed was 3 feet per minute, resulting in a furnace residence time of about 10 minutes. The treated cloth contained 1.2 weight percent chromium sesquioxide in the form of a uniform, well adherent coating.

EXAMPLE 8 Twenty-five pounds of ammonium dichromate were added to 25 gallons of deionized water contained in a large polyethylene-lined drum. One-hundred pounds of oxalic acid dihydrate were then added slowly over a period of about one hour. The reactants were completely dissolved and reacted within approximately 30 minutes after addition of the final increment of oxalic acid dihydrate, and the resulting solution was hot, and green in color. This green solution was immediately applied to a roll of C1004853 Refrasil cloth in the apparatus of FIG. I. C-100-48-53 Refrasil is identical to C-100-48 Refrasil, except that its nominal width is 53 inches. The furnace was at a temperature of 1700 F. Processing speed was 3 feet per minute, resulting in a furnace residence time of about 10 minutes. The treated cloth contained 2.4 weight percent chromium sesquioxide in the form of a uniform, well adherent coating.

EXAMPLE 9 A sample of C-l00-48 Refrasil cloth was treated in accordance with the prior art procedure to apply a 1.3 weight percent chromium oxide coating to the cloth by successive treatment in baths of chromium sulfate and ammonium hydroxide to cause precipitation of a chromium hydroxide coating on the silica. The coated silica cloth was fired to pyrolytically decompose the hydroxide to chromium sesquioxide. The coating was non-uniform and excessively dusty. Samples of the treated cloth of Examples 7 and 8 (present invention process: 1.2% and 2.4% Cr O respectively) and Example 9 (prior art process; 1.3% -Cr. O' were tested] and the results compared to those obtained with an untreated sample of C- 100-48 Refrasil. The present invention process samples consistently yielded the highest warp direction breaking strength; from 10% to 35% and from 25% to 50% higher than the untreated and prior art process samples, respectively. All of the chromium sequioxide coated samples exhibited significantly higher flex-abrasion resistance than the untreated sample, with the present invention process samples showing the highest flex-abrasion resistance; this may be attributed to the chromium sesquioxide acting as a lubricant, with the superiority of the present invention process samples being due to the greater chromium sesquioxide penetration and the coating uniformity afforded by the present invention process. The present invention process 2.4% Cr O sample further shows a thermal conductivity at 1000 F. in air about 4% lower than the prior art process sample, and about 8% lower than the untreated sample as shown in FIG. 2. This decrease in thermal conductivity as compared to the prior art process sample is at least partially due to the higher chromium sesquioxide content which can be effected by the present invention process, without resulting in an unacceptably dusty coating.

Further testing was conducted after the samples were subjected to an extended high temperature exposure of 100 hours at 2100 F. in air. The present invention process samples still exhibited the highest warp direction breaking strength: 35% and 100% higher than the untreated and prior art process samples, respectively. In addition, the ultimate elongation, which is inversely related to brittleness and provides a comparative measure thereof, of the present invention process. 2.4% Cr O sample showed no decline from its original value. That of the present invention process 1.2% C and prior art process samples declined slightly, while that of the untreated sample declined drastically. Accordingly, at the two extremes the present invention process 2.4% Cr O sample was soft and flexible, while the untreated sample was hard and brittle, and broke when flexed.

Cost analysis indicates that, based on the material and labor savings realized, use of the process of the present invention results in a significant reduction in production cost.

In summary, the process of the present invention is a substantial advance in the art since it provides an improved product at lower cost; it eliminates the use of objectionable and noxious chemicals and further eliminates the use of separate dipping tanks, stirring, and heating in the multiple stage wet treatment as practiced previously. The product as produced by the method of this invention has increased chromium oxide penetration, yet is relatively dust-free. The coating is extremely uniform.

The product has higher warp strength even after extended exposure to high temperatures, and greater abrasion resistance. The thermal conductivity is slightly lowered and the wettability by impregnating resins such as phenolics is improved.

It is to be understood that only preferred embodiments of the invention have been described and that numerous substitutions, modifications and alterations are all permissible without departing from the spirit and scope of the invention as defined in the following claims.

What is claimed is:

1. A method of treating siliceous articles comprising the steps of:

dissolving in solvent a dichromate salt and oxalic acid in an equivalent ratio of at least 5:1 of acid to dichromate to form a solution;

stoichiometrically reacting said salt and acid to form a solution containing a Werner complex of oxalic acid and said salt in said solvent;

applying said solution to a surface of the article as a treating agent to form a coating of said complex on the surface; and

thermally decomposing said complex on the surface at a temperature from 1000 F. to 3000 F. for a time sufiicient to convert the complex to a bonded layer of chromium oxide on said surface.

2. A method according to claim 1 wherein said salt is ammonium dichromate.

3. A method according to claim 2 wherein said complex is ammonium dioxalatodiaquochromate.

4. A method according to claim 1 wherein the article is comprised of high silica content fibers.

5. A method according to claim 4 wherein said fibers contain at least 96% silica.

6. A method according to claim 5 wherein said fibers are treated to contain 1 to 6% chromium (III) oxide.

7. A method according to claim 1 wherein said thermal decomposition is conducted in an oxygen containing atmosphere.

8. A method according to claim 1 wherein said ternperature is from 1500 F. to 2000 F.

9. An article produced according to the method of claim 1.

10. An article according to claim 9 comprised of high silica content fibers.

11. An article according to claim 10 wherein said fibers are woven into a yarn.

12. An article according to claim 11 wherein said yarn is woven into a fabric.

13. An article according to claim 10 wherein said fibers are formed into a batt.

14. A method improving the high temperature stability of articles formed of high silica content fibers comprising the steps of:

dissolving in water ammonium dichromate and oxalic acid in which the equivalent ratio of acid to dichromate is at least 5:1;

stoichiometrically reacting said dichromate and acid to form an aqueous solution containing ammonium dioxalatodiaquochromate complex; impregnating the surface of the article with said aqueous solution of ammonium dioxalatodiaquochromate complex under conditions to provide a concentration of chromium (III) oxide by weight of 1 to 6 percent on said article after firing;

maintaining contact between said solution and said article for a time sufficient to coat the surfaces of said fibers; and

firing the article at a temperature of from 1500 F. to

2000 F. in an oxygen containing atmosphere for a time sufiicient to decompose and convert said complex to a layer of chromium (I-II) oxide bonded to said fiber surface.

15. A fibrous siliceous article having at least one surface portion containing a layer comprising ammonium dioxalatodiaquochromate.

16. An article according to claim 15 comprised of a high silica content roving.

17. An article according to claim 16 wherein said roving is woven into a fabric.

18. An article according to claim 15 in which said ratio is at least 8:1.

References Cited UNITED STATES PATENTS 3,232,782 2/ 1966 Shannon 117126 GF FOREIGN PATENTS 1,047,932 11/1966 Great Britain.

646,835 8/1962 Canada 260-4385 C OTHER REFERENCES Wendland et al.: The Thermal Decomposition of Metal Complexes-I, 21 J. Inorg. & Nucl. Chem. (1961), pp. '69, 7346.

MURRAY KATZ, Primary Examiner D. COHEN, Assistant Examiner US. Cl. X.R.

ll7126 GF, GQ, 123 A; 260-4385 C 

